Change XO to XXO in many more places, with no effects at all...

... because the two macros have the same expansion, and are both checked for
in the --keyword arguments passed to msgfmt by locale/update_po_files.sh.

This commit makes ONLY such changes, and comments in Internat.h.  It is big
but quite harmless.

The intention is to introduce a type distinction in a later release, by defining
XXO differently.  XXO is used where & characters in strings (for hotkeys of menu
items or control prompts) are permitted, XO where not.

src/ProjectAudioManager.cpp

@@ -900,7 +900,7 @@ You are saving directly to a slow external storage device\n\
 "
          ),
          false,
-         XO("Turn off dropout detection"));
+         XXO("Turn off dropout detection"));
       }
 
       auto &history = ProjectHistory::Get( project );